Linear Polyene Electronic Structure and Potential Surfaces

Polaritonic Chemistry: Hindering and Easing Ground State Polyenic Isomerization via Breakdown of σ-π Separation

The ground state conformational isomerization in polyenes is a symmetry allowed process. Its low energy barrier is governed by electron density transfer from the formal single bond that is rotated to the nearby formal double bonds. Along the reaction pathway, the transition state is therefore destabilized. The rules of polaritonic chemistry, i.e., chemistry in a nanocavity with reflecting windows, are barely beginning to be laid out. The standing electric field of the nanocavity couples strongly with the molecular wave function and modifies the potential energy curve in unexpected ways. A quantum electrodynamics approach, applied to the torsional degree of freedom of the central bond of butadiene, shows that formation of the polariton mixes the σ-π frameworks thereby stabilizing/destabilizing the planar, reactant-like conformations. The values of the fundamental mode of the cavity field used in the absence of the cavity do not trigger this mechanism.

Solvent-Dependent Characterization of Fucoxanthin through 2D Electronic Spectroscopy Reveals New Details on the Intramolecular Charge-Transfer State Dynamics

The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.

Carotenoid Nuclear Reorganization and Interplay of Bright and Dark Excited States

We report quantum chemical calculations using multireference perturbation theory (MRPT) with the density matrix renormalization group (DMRG) plus photothermal deflection spectroscopy measurements to investigate the manifold of carotenoid excited states and establish their energies relative to the bright state (S₂) as a function of nuclear reorganization. We conclude that the primary photophysics and function of carotenoids are determined by interplay of only the bright (S₂) and lowest-energy dark (S₁) states. The lowest-lying dark state, far from being energetically distinguishable from the lowest-lying bright state along the entire excited-state nuclear reorganization pathway, is instead computed to be either the second or first excited state depending on what equilibrium geometry is considered. This result suggests that, rather than there being a dark intermediate excited state bridging a non-negligible energy gap from the lowest-lying dark state to the lowest-lying bright state, there is in fact no appreciable energy gap to bridge following photoexcitation. Instead, excited-state nuclear reorganization constitutes the bridge from S₂ to S₁, in the sense that these two states attain energetic degeneracy along this pathway.

Correlated Electronic Properties of a Graphene Nanoflake: Coronene

We report studies of the correlated excited states of coronene and substituted coronene within the Pariser–Parr–Pople (PPP) correlated π-electron model employing the symmetry-adapted density matrix renormalization group technique. These polynuclear aromatic hydrocarbons can be considered as graphene nanoflakes. We review their electronic structures utilizing a new symmetry adaptation scheme that exploits electron-hole symmetry, spin-inversion symmetry, and end-to-end interchange symmetry. The study of the electronic structures sheds light on the electron correlation effects in these finite-size graphene analogues, which diminishes going from one-dimensional to higher-dimensional systems, yet is significant within these finite graphene derivatives.

Single, Double Electronic Excitations and Exciton Effective Conjugation Lengths in π-Conjugated Systems

The 21Ag and 11Bu excited states of two prototypical π-conjugated compounds, polyacetylene and polydiacetylene, are investigated with the recently developed particle-particle random phase approximation (pp-RPA) method combined with the B3LYP functional. The polymer-limit transition energies are estimated as 1.38 and 1.72 eV for the 21Ag and 11Bu states, respectively, from an extrapolation of the computed excitation energies of model oligomers. These values increase to 1.95 and 2.24 eV for the same transitions when ground-state structures with ∼33% larger bond length alternation are adopted. Applying the pp-RPA to the vertical excitation energies in oligodiacetylene, the polymer-limit transition energies of the 21Ag and 11Bu states are computed to be 2.06 and 2.28 eV, respectively. These results are in good agreement with experimental values or theoretical best estimates, indicating that the pp-RPA method shows great promise for understanding many photophysical phenomena involving both single and double excitations.

Polyacetylene: Myth and Reality

Polyacetylene, the simplest and oldest of potentially conducting polymers, has never been made in a form that permits rigorous determination of its structure. Trans polyacetylene in its fully extended form will have a potential energy surface with two equivalent minima. It has been assumed that this results in bond length alternation. It is, rather, very likely that the zero-point energy is above the Peierls barrier. The experimental studies that purport to show bond alternation are reviewed and shown to be compromised by serious experimental inconsistencies or by the presence, for which there is considerable evidence, of finite chain polyenes. In this view, addition of dopants results in conductivity by facilitation of charge transport between finite polyenes. The double minimum potential that necessarily occurs for polyacetylene, if viewed as the result of elongation of finite chains, originates from admixture of the 1¹A_g ground electronic state with the 2¹A_g excited electronic singlet state. This excitation is diradical (two electron) in character. The polyacetylene limit is an equal admixture of these two ¹A_g states making theory intractable for long chains. A method is outlined for preparation of high molecular weight polyacetylene with fully extended chains that are prevented from reacting with neighboring chains.

Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wave functions of the optical singlets, the lowest triplet exciton, and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against earlier and very recent experimental transient absorptions in the near- and mid-infrared, we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2¹A_g^- in linear polyenes.

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have a stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.

Covalent Excited States of Polyenes C2nH2n+2 (n = 2-8) and Polyenyl Radicals C2n-1H2n+1 (n = 2-8): An Ab Initio Valence Bond Study

The ab initio valence bond (VB) methods, VBSCF and VBCI, are applied to the ground states and the covalent excited states of polyenes C2nH2n+2 (n = 2-8) and polyenyl radicals C2n-1H2n+1 (n = 2-8). The excitation energy gap was computed at the ab initio VB level, which is in good agreement with the semiempirical VB method, VBDFT(s), and the experimental values as well as with the molecular orbital theory based methods, CASPT3 and MRCI. The ab initio VB wave functions of systems are also in very good agreement with those of the VBDFT(s) method, even though the former is based on the ab initio VB scheme while the latter is a semiempirical Hückel type method, in which no orbital optimization procedure is performed. The computational results show that the ab initio VB method is capable now of providing numerical accuracy not only for bond forming and breaking processes, as shown in the past, but also for excitation energies, as shown here. In addition, the computational results validate the efficiency of the VBDFT(s) method, which is a simple VB model with less computational effort but which provides intuitive insights into the excited states of conjugated molecules.

Strong Photophysical Similarities between Conjugated Polymers and J-aggregates

The photophysical properties of emissive conjugated polymer (CP) chains are compared to those of linear J-aggregates. The two systems share many properties in common, including a red-shifted absorption spectrum with increasing chain/aggregate length, enhanced radiative decay rates (superradiance) relative to a single monomer/molecule, and several vibronic signatures involving the vinyl-stretching mode common to many conjugated molecules. In particular, the scaling of the 0-0/0-1 photoluminescence ratio and radiative decay rate with the inverse square root of temperature in red-phase polydiacetylene is also characteristic of linear, disorder-free J-aggregates. The strong photophysical resemblance is traced to the excitonic band structure; in one-dimensional direct band gap semiconductors as well as J-aggregates, the exciton band curvature is positive at the gamma point (k = 0).

25th anniversary article: Design of polymethine dyes for all-optical switching applications: guidance from theoretical and computational studies

All-optical switching--controlling light with light--has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π-conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π-conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

Demystifying the dendralenes

We present herein an overview of our ongoing studies with dendralenes. The first synthetic routes to this fundamental family of compounds have revealed long-hidden secrets in hydrocarbon chemistry and set the scene for synthetic and materials chemistry applications.

Energetics and dynamics of the low-lying electronic states of constrained polyenes: implications for infinite polyenes

Steady-state and ultrafast transient absorption spectra were obtained for a series of conformationally constrained, isomerically pure polyenes with 5-23 conjugated double bonds (N). These data and fluorescence spectra of the shorter polyenes reveal the N dependence of the energies of six (1)B(u)(+) and two (1)A(g)(-) excited states. The (1)B(u)(+) states converge to a common infinite polyene limit of 15,900 ± 100 cm(-1). The two excited (1)A(g)(-) states, however, exhibit a large (~9000 cm(-1)) energy difference in the infinite polyene limit, in contrast to the common value previously predicted by theory. EOM-CCSD ab initio and MNDO-PSDCI semiempirical MO theories account for the experimental transition energies and intensities. The complex, multistep dynamics of the 1(1)B(u)(+) → 2(1)A(g)(-) → 1(1)A(g)(-) excited state decay pathways as a function of N are compared with kinetic data from several natural and synthetic carotenoids. Distinctive transient absorption signals in the visible region, previously identified with S* states in carotenoids, also are observed for the longer polyenes. Analysis of the lifetimes of the 2(1)A(g)(-) states, using the energy gap law for nonradiative decay, reveals remarkable similarities in the N dependence of the 2(1)A(g)(-) decay kinetics of the carotenoid and polyene systems. These findings are important for understanding the mechanisms by which carotenoids carry out their roles as light-harvesting molecules and photoprotective agents in biological systems.

Excitation spectra of cation and anion radicals of several unsaturated hydrocarbons: symmetry adapted cluster-configuration interaction theoretical study

Excitation spectra of cation and anion radicals of unsaturated hydrocarbons, hexatriene(±), octatetraene(±), cyclopentadiene(+), 1,3-cyclohexadiene(+), and naphthalene(±), were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method. The calculated results reasonably reproduced the experimental spectra observed by one of the authors (T.S.) and gave reasonable assignments. The two bands observed in the experimental spectra were assigned to π-π(SOMO) and π(SOMO)-π* for the cation radicals and π*(SOMO)-π* and π-π*(SOMO) for the anion radicals, in order of increasing energy, except for naphthalene(±). The four bands of naphthalene(+) originated from π-π(SOMO), π-π(SOMO), π(SOMO)-π*, and π(SOMO)-π* and those of naphthalene(-) originated from π*(SOMO)-π*, π*(SOMO)-π*, π-π*(SOMO), π*(SOMO)-vπ*, and π-π*(SOMO), in order of increasing energy. The SOMO orbitals were involved in the intense bands of both cation and anion radicals. Moreover, the ionization energies (IEs) and electron affinities (EAs) of these hydrocarbons were in good agreement with the experimental values, whereas the EAs of hexatriene and octatetraene were predicted to be negative and positive, respectively. The calculated IE + EA values were nearly constant for the three π-π* pairing states of hexatriene(±), octatetraene(±), and naphthalene(±), indicating that the pairing theorem is valid even at the SAC-CI level.

Two-Photon Absorbing Organic Chromophores for Optical Limiting

Strong optical limiting and large two-photon absorptivities are reported for a class of bisdonor diphenylpolyene derivatives with varying polyene bridge lengths. These molecules exhibit strong optical limiting using nanosecond pulses over a broad spectral range. Bis(diphenylamino)stilbene exhibits a 90 nm red shift of its optical limiting band, and only a minimal shift of about 13 nm of its lowest one-photon electronic absorption band relative to bis(di-n-butylamino)stilbene. This suggests a potential for broadband optical limiting with high transparency using mixtures of such compounds. Pulse width dependent nonlinear transmission measurements suggest that two-photon pumped excited-state absorption contributes significantly to the limiting of nanosecond pulses.

Electronic excitations in long polyenes revisited

We apply the valence shell model OM2 [W. Weber and W. Thiel, Theor. Chem. Acc. 103, 495, (2000)] combined with multireference configuration interaction (MRCI) to compute the vertical excitation energies and transition dipole moments of the low-energy singlet excitations in the polyenes with 4 ≤ N ≤ 22π-electrons. We find that the OM2/MRCI descriptions closely resemble those of Pariser-Parr-Pople (PPP) π-electron models [P. Tavan and K. Schulten, Phys. Rev. B 36, 4337, (1987)], if equivalent MRCI procedures and regularly alternating model geometries are used. OM2/MRCI optimized geometries are shown to entail improved descriptions particularly for smaller polyenes (N ≤ 12), for which sizeable deviations from the regular model geometries are found. With configuration interaction active spaces covering also the σ- in addition to the π-electrons, OM2/MRCI excitation energies turn out to become smaller by at most 0.35 eV for the ionic and 0.15 eV for the covalent excitations. The particle-hole (ph) symmetry, which in Pariser-Parr-Pople models arises from the zero-differential overlap approximation, is demonstrated to be only weakly broken in OM2 such that the oscillator strengths of the covalent 1B(u)(-) states, which artificially vanish in ph-symmetric models, are predicted to be very small. According to OM2/MRCI and experimental data the 1B(u)(-) state is the third excited singlet state for N < 12 and becomes the second for N ≥ 14. By comparisons with results of other theoretical approaches and experimental evidence we argue that deficiencies of the particular MRCI method employed by us, which show up in a poor size consistency of the covalent excitations for N > 12, are caused by its restriction to at most doubly excited references.

Absorption spectra of α,ω-diphenylhexadecaoctaene and shorter diphenylpolyenes

Absorption spectrum of α,ω-diphenylhexadecaoctaene has been measured along with those of α,ω-diphenylpolyenes with one to seven double bonds in the polyene chain in carbon tetrachloride at room temperature. Extrapolation of the observed vibrational frequencies in the 1(1)Bu state measured as a function of polyene double bond number provides the CC and CC stretching frequencies of 1520±20 and 1080±20 cm(-1), respectively, for diphenylpolyenes with infinite polyene chain length.

COUPLED-CLUSTER EQUATION OF MOTION STUDY FOR THE ELECTRONIC AND OPTICAL PROPERTIES OF CONJUGATED SYSTEMS

We have implemented the Coupled-Cluster Equation of Motion (CC-EOM) with single and double excitations coupled with semiempirical parameterizations, Pariser–Parr–Pople model and INDO Hamiltonians, to investigate the optical and nonlinear optical properties, electronic structures and the excited states properties for the conjugated polymers. The semiempirical parameters allow us to study the conjugated systems with extensive sizes. Firstly, by comparing with the quasi-exact Density Matrix Renormalization Group theory for the 1D conjugated chain, we find that the CC-EOM approach can give satisfactory results for both the ground state and the excited states energies. We demonstrate that our approach can be adopted to evaluate linear and nonlinear response. We find that both the real and imaginary parts of the third-order polarizability can be solved either for static and dynamic responses. Then we apply the CC-EOM approach to study the optical signatures for the polarons in conjugated polymers. We have established a solid relationship between the rigidity of a polymer and its optical signature of the polarons.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.

Excitation energy calculation of conjugated hydrocarbons: a new Pariser-Parr-Pople model parameterization approaching CASPT2 accuracy

A new parameterization for the Pariser-Parr-Pople (PPP) model for conjugated hydrocarbons is proposed in this work. The distance-dependence of PPP parameters are obtained from CASPT2 ground state and low-lying excited state energies of ethylene and its cation at various C-C single bond lengths and are fitted to a set of carefully chosen mathematical functions. Our new PPP model is applied to the calculation of vertical singlet-triplet energy gaps and the excitation energies for low-lying π→π(*) valence excitations in various π-conjugated molecules. Results with the new PPP model are consistently better than the standard PPP model in use. It often surpasses density functional theory and single-reference excited state methods such as configuration interaction singles or time-dependent density functional theory in terms of its accuracy and agrees reasonably well with high-level theories or experiments.